1. Field of the Invention
This invention relates generally to sensor signal processing, and more particularly to the field of array motion compensation. A particular use of the invention is in movable linear arrays which are subject to lateral distortion.
2. Prior Art
A linear array is a linear distribution of sensors, or other type of emitting/receiving device, in which there are a multitude of receiving/radiating elements, each arranged in a straight line. When the received or emitted signal is represented in frequency domain and properly phased or time delayed for arrivals from or emissions to a particular direction, the resulting interference produces a maximum response in that direction. This maximum response is called a beam in that direction. Beams may be formed in multiple directions simultaneously by a process known as a beamformer. The linear array can be designed to operate either actively or passively.
The direction and intensity of the beams is determined by the beamformer which controls the output of the beams in either the time domain or the frequency domain. A time domain beamformer inserts a time delay into the signal from/to each radiating element. A frequency domain beamformer shifts the phase of the signal from/to each radiating element. Frequency domain beamformers are gaining in acceptance in the art due to the increased abundance and speed of Fast Fourier Transform (FFT) electronic devices.
An interference pattern is created by the intersection of the modulated signal from each of the radiating elements, and these lines of interference are the antenna beams.
A fixed linear array can achieve a high degree of beam pointing accuracy, because there is no lateral distortion of the array. However, when the array is in motion, the linear symmetry or shape of the array is distorted and the beam pointing accuracy significantly diminishes. A towed SONAR array is an example of a moving linear array. In a towed SONAR array, receiving elements are strung linearly along the length of a cable, which is towed behind a ship. An array can generate a very high number of beams, each projecting outward from the array. A towed linear array operating passively is extremely useful, as the beams can simultaneously identify and localize numerous ocean targets, such as enemy ships or submarines, in various positions relative to the tow ship, without giving away the tow ship location.
A significant problem with towed arrays is the difficulty in maintaining the linear symmetry of the array. As the cable moves through the water, the ocean currents and the even velocity and steering of the tow ship causes distortions in the shape of the array. These distortions diminish the accuracy and utility of the towed array, because the actual beam pointing direction is different from the calculated direction. There currently exists no method of compensating for this distortion, and the reduction in accuracy is generally accepted as a condition of the device.
It would be highly desirable to develop a method for compensating for the lateral motion of a linear array using a frequency domain beamformer. The present invention was developed to address this need.